1. Field of the Invention
This invention relates generally to optical apparatus in semiconductor technology, and more particularly, to a test monitor for use in focusing an image on a semiconductor wafer.
2. Discussion of the Related Art
Typically, an optical system 30 (FIG. 1) used for patterning photoresist 32 on a semiconductor wafer 34 comprises a light source 36, a mask or reticle 38 having opaque lines 40 and transparent portions 42, and a lens 44, the light from the light source 36 passing through the transparent portions 42 of the mask/reticle 38 through the lens 44 and to the photoresist 32, with light being blocked from reaching the lens 44 (and photoresist 32) by the opaque lines 40 of the 38 mask/reticle.
As is well known, there is a need to position the wafer 34 at a proper distance with respect to the lens 44 so that fall on the photoresist 32 of the wafer 34 will be in the plane of best focus.
Generally, prior to actual fabrication of semiconductor devices, a test focus monitor in the form of for example a reticle is used as part of the overall system to achieve proper focus of the image on the wafer. An example of such a monitor is shown and described in the paper entitled xe2x80x9cNew Phase Shift Ratings For Measuring Aberrationsxe2x80x9d, by Hiroshi Nomura, published by SPIE, dated Feb. 27, 2001, which is herein incorporated by reference. FIGS. 2-4 herein show a monitor 50 configured as shown in FIGS. 3 and 5 of that paper. The monitor 50 is made up of a quartz base 52 which is transparent to light, and which has a plurality of parallel, opaque, spaced apart lines 54 on the base 52, the lines 54 having a first set of adjacent ends 55, and a second, opposite set of adjacent ends 56. The area between each adjacent pair of lines 54 is transparent to light and is made up of regions 58 which pass light therethrough without changing the phase thereof, and regions 60 which pass light therethrough which shift the phase of such light by 90xc2x0 (the phase shifting caused by recesses 62 in the base 52xe2x80x94see FIGS. 3 and 4 and the above cited paper). Each of the lines 54 has a region 58 and a region 60 which are aligned along and on one side thereof, and a region 58 and a region 60 which are aligned along and on the opposite side thereof. Each of the lines 54 has a region 58 on one side thereof opposite a region 60 on the other side thereof, these regions 58, 60 running from end 55 of that line to adjacent to the middle thereof, and furthermore, each of the lines 54 has a region 60 on the one side thereof opposite a region 58 on the other side thereof, these regions 60, 58 running from end 56 to adjacent the middle thereof.
FIGS. 3 and 4 are views similar to that shown in FIG. 1, but incorporating the monitor 50 of FIG. 2 as a part of the system 30. FIG. 3 includes a sectional view of the monitor 50 taken along the line 3xe2x80x943 of FIG. 2, showing a cross-section of the upper area 50A of the monitor 50, while FIG. 4 includes a sectional view of the monitor 50 taken along the line 4xe2x80x944 of FIG. 2, showing a cross-section of the lower area 50B of the monitor 50. As will be seen, with reference to the upper area 50A of the monitor 50 FIG. 3), moving the wafer 34 and lens 44 relatively together and apart causes the shadows 64A, 64B, 64C formed on the photoresist 32 of the wafer 34 (formed by the opaque lines 54) to shift (downward as the wafer 34 and lens 44 are moved relatively further apart). Mile, with reference to the lower area 50B of the monitor 50 (FIG. 4), moving the wafer 34 and lens 44 relatively together and apart causes the shadows 64D, 64E, 64F formed on the photoresist 32 of the wafer 34 to sit (upward as the wafer 34 and lens 44 are moved relatively further apart). The doted lines 66 in FIGS. 3 and 4 indicate the traverse of the shadows 64A, 64B, 64C, 64D, 64E, 64F as the wafer 34 is so moved relatively toward and away from the lens 44.
These paths are plotted m FIG. 5, and the intersections thereof indicate the best focus of the image on the wafer 34.
FIG. 6 includes FIGS. 6A-6F which are views taken along the lines 6Axe2x80x946A, 6Bxe2x80x946B, 6Cxe2x80x946C, 6Dxe2x80x946D, 6Exe2x80x946E, and 6Fxe2x80x946F of FIGS. 3 and 4. With the wafer 34 and lens 44 closest together as shown in FIGS. 3 and 4, FIGS. 6A and 6D show the simultaneous positions of the shadows 64A-64F on the photoresist 32 determined by the respective areas 50A, 50B of the monitor 50. With the wafer 34 and lens 44 so positioned relative to each other, the photoresist 32 is exposed to light from the light source 36 and is then developed to determine photoresist lines which corresponds to the positions of the shadows 64A-64F. As will be seen, the lines of FIGS. 6A and 6D are misaligned As the wafer 34 and lens 36 are moved relatively further apart to an intermediate position as shown in FIGS. 3 in 4, FIGS. 6B and 6E show the simultaneous positions of the shadows 64A-64F on the photoresist 32 determined by the respective areas 50A, 50B of the monitor 50. Again, the photoresist 32 is exposed to light from the light source 36 and is then developed to determine photoresist lines which correspond to the positions of the shadows 64A-64F. As will be seen, the lines of FIGS. 6B and 6E are substantially in alignment. Then, as the wafer 34 and lens 44 are moved relatively further apart, i.e., to their most far apart positions as shown in FIGS. 3 and 4, FIGS. 6C and 6F show the simultaneous positions of the shadows 64A-64F on the photoresist 32 determined by their respective areas 50A, 50B of the monitor 50. Again with the wafer 34 and lens 44 so positioned relative to each other, the photoresist 32 is exposed to a light from the light source 36 and is then developed to determine photoresist lines which correspond to the positions of the shadows 64A-64F. As will be seen, the lines of FIGS. 6E and 6F are misaligned.
It will be seen that changing the distance between the lens 44 and wafer 34 causes the shadows 64A-64C to move further in and out of alignment with the shadows 64D-64F. The process of moving the lens 44 and wafer 34 Lively closer together and further apart, along with the corresponding exposure and development of the photoresist 32 accompanying each adjustment, is repeated until the lines formed in the photoresist 32 are substantially straight. This is illustrated in FIG. 6 of the above cited paper.
While such an approach is useful, only a relatively coarse reading of focus is achievable. For example, with reference to FIG. 6 of the above cited paper, only a small shift in the pattern from top to bottom is shown over a range of 400 nm of rive movement between the wafer 34 and lens 44. With device dimensions continually being reduced, there is a need to achieve a proper reading of focus within a much smaller range of lens-wafer relative movement, for example, 200 nm or less.
The Benchmark Technologies Incorporated Phase Shift Focus Monitor Test Reticle 100 (FIG. 7) uses phase shifting to produce images which shift according to the magnitude of defocus. In this apparatus 100, a quartz body 102 which is transparent to light has thereon opaque, for example chrome, lines 104, 106, 108, 110 which define an outline 112 in the shape of a square. The body 102 also has thereon opaque, for example chrome, lines 114, 116, 118, 120 that define an outline 122 in the shape of a square, which is centrally positioned relative to and within the square 112. The regions 124 of the quartz body 102 allow light to be transmitted therethrough without changing the phase thereof, while the regions 126, recessed as described above, allow light to be transmitted therethrough while changing the phase thereof by 90xc2x0. It will be seen that with this configuration, each opaque line is positioned between phase shifted and unshifted regions of the body.
With the reticle 100 of FIG. 7 used in the apparatus of FIG. 1, variation in the distance between the lens 44 and the wafer 34 causes the images 128, 130 of the square 112 and the square 122 to shift relative to each other as shown in FIGS. 8A, 8B and 8C. That is, with the distance between the lens 44 and wafer 34 providing proper focus, the images 128, 130 formed by the square 112 and the square 122 correspond to the positioning of the outlines 112, 122 on the reticle 100 forming these images, i.e., the image 130 of the square on the wafer 34 is centrally located within the image of the square 128 on the wafer 34 (FIG. 8B). Decreasing the distance between the lens 44 and the wafer 34 causes the image 128 formed by the outline 112 to shift leftward and upward while the image 130 formed by the outline 122 shifts rightward and downward (FIG. 8A), indicating defocus. Increasing the distance between the lens 44 and wafer 34 causes the image 128 formed by the outline 112 to shift rightward and downwardly while the image 130 formed by the outline 122 shifts leftward and upward until the image 130 is centrally located within the image 128 (see FIG. 8B), indicating that the image is in focus on the wafer 34. Further movement of the lens 44 and wafer 34 apart causes the image 128 formed by the outline 112 to further move downward and rightward, while the image 130 formed by the outline 122 further moves upward and leftward (FIG. 8C), indicating defocus.
Similar to the above-described system, only a relatively coarse reading of focus is achievable. For example, the shift in positions of the images when going from FIG. 8A to FIG. SC occurs over a range of 800 nm relative movement between the wafer 34 and lens 44. With device dimensions continually being reduced, there is a need to achieve a proper reading of focus within a much smaller range of lens-wafer relative movement, for example 200 nm or less.
The present invention is an optical tool including a tool body that is transparent to light. Pluralities of opaque, parallel lines on the body form a first outline in the shape of square. Additional pluralities of opaque, parallel lines on the body form an outline in the shape of the square that is centrally located relative to and within the first-mentioned square. Each pair of adjacent lines has therebetween a first region which allows transmission of light therethrough without changing phase thereof and a second region alongside the first region which allows transmission of light therethrough while shifting the phase thereof by 90xc2x0. The regions are laid out so that the images of the outlines formed by a lens on an object shift a subs amount upon relative movement between the lens and object.